THE ROLE OF EXOGENOUS P53 AND E6 ONCOPROTEINS IN IN-VITRO TRANSFORMATION BY BOVINE PAPILLOMAVIRUS TYPE-4 (BPV-4) - SIGNIFICANCE OF THE ABSENCE OF AN E6 ORF IN THE BPV-4 GENOME

Bovine papillomavirus type 4 (BPV-4) does not possess an E6 ORF, The E 6 oncoprotein of human papillomavirus (HPV) binds and degrades the tum our suppressor protein p53, thus contributing to tumour progression. S ince BPV-LF lacks E6, it is unknown how the virus evades the tumour su ppressor properties of p53 in the induction of tumours of the gastroin testinal tract, Mutations in the p53 gene have been detected both in p apillomas and carcinomas, suggesting that p53 dysfunction plays a part in these neoplasias, BPV-4 can transform primary foetal bovine cells (PalFs) in cooperation with an activated res gene, but the transformed cells are neither immortal nor tumorigenic, Co-transfection with the HPV-16 E6 (16E6) ORF confers immortality but not tumorigenicity. To in vestigate the role of p53 in BPV-4 cell transformation in vitro, we tr ansfected PalFs and p53-null mouse fibroblasts with BPV-4 DNA in combi nations with ras, 16E6 ORF and mutant (V143A) p53 cDNA, Transfection o f PalFs with BPV-4 DNA, ras and mutant p53 led to cell immortalization , indicating that 16E6 and mutant p53 are functionally equivalent in c onferring immortality, However, cotransfection of PalFs with BPV-4 DNA , ras, and both mutant p53 cDNA and 16E6 ORF resulted in cells which w ere fully transformed to tumorigenicity. In p53-null mouse fibroblasts , BPV-4 DNA induced transformation by itself, but the transformed cell s were incapable of suspension growth, The cotransfection of BPV-4 DNA with 16E6 ORF produced many more transformed colonies and the cells w ere capable of growing in suspension. In this system, therefore, 16E6 confers anchorage-independence to BPV-4-transformed cells in a p53-ind ependent fashion.